I am not sure why the UGUI actions are not part of the main PM install. It's not utterly unseemly to have to use the ecobrowser to get them, but the UI is sort of a basic pillar of Unity, so I can't figure why they simply aren't there.

The reason why it's not there yet, is that PlayMaker was supported until 1.8 version older Unity versions that did not had the new UI system. now that PlayMaker 1.8 minimum Unity version has the new UI, it will become possible to include UI in the PlayMaker package.

Also, given the long history of trial and error with Unity UI systems ( they went with I believe 5 different attempts...) a lot of project still rely on other UI system, like Ngui, which is still a very good and valid UI system, so not everyone is using Unity UI, and so it's a balance to get. Now, it seems that the new UI system is here to stay so it starts to make sense to include it by default.

The addon and Ecosystem should not be underestimated for a truly taylored project and avoids clogging your project with unnecessary files, this needs to be taken in consideration as well.